The present disclosure provides novel addition-fragmentation oligomers for use in low-stress polymerizable compositions. Free-radical polymerization is typically accompanied by a reduction in volume as monomers are converted to polymer. The volumetric shrinkage produces stress in the cured composition, leading to microcracks and deformation. Stress transferred to an interface between the cured composition and a substrate can cause failure in adhesion and can affect the durability of the cured composition.
The crosslinking oligomers of this disclosure provide stress relief by including labile crosslinks that can cleave and reform during the polymerization process. Crosslink cleavage may provide a mechanism to allow for network reorganization, relieve polymerization stress, and prevent the development of high stress regions. The instant crosslinking oligomer may further provide stress relief by delaying the gel point, the point at which the polymerizable composition transitions from a viscous material to a viscoelastic solid. The longer the polymerizable mixture remains viscous, the more time available during which material flow can act to alleviate stress during the polymerization process.
Curable polymeric materials are used in a wide variety of dental applications, including restoratives, cements, adhesives, and the like. Often, such materials shrink upon curing. This is particularly problematic when the material is in a constrained environment, as in a dental filling or restorative, for example. Dimensional changes upon shrinkage while in a constrained environment can generate a strain within the material that is typically converted into a stress on the surrounding environment (e.g., tooth). Such forces can result in interfacial failures between the tooth and the polymeric material resulting in a physical gap and subsequent microleakage into the tooth cavity. Alternatively, such forces can lead to fractures within the tooth and/or the composite.
Generally, conventional processes of curing polymeric dental materials involve a composite held in place on an oral surface with an adhesive and involve curing the adhesive and then subsequently curing the composite material. More specifically, conventional methods utilize one or more of the following steps: surface treatment of the tooth (e.g., etching, priming), application of a curable adhesive to the tooth surface, curing of the adhesive, placement of a composite material (e.g., restorative) on the cured adhesive, and curing of the composite material. There is a need for dental materials, e.g., dental adhesives and dental composites that reduce the amount of stress placed on the dental material and the surrounding environment during or after curing.
Although various curable dental compositions have been described, industry would find advantage in compositions having improved properties such as reduced stress deflection and/or reduced shrinkage while maintaining sufficient mechanical properties and depth of cure.